1. Field of the Invention
The present invention relates in general to apparatus for treating air, in particular the air in aircraft cabins.
2. Description of Related Art
The quality of air in an aircraft cabin is a problem that involves numerous parameters. Thus, the air must comply with maximum acceptable concentrations both for microparticles and for microorganisms, so as to ensure that the air is well tolerated by human beings.
Furthermore, mainly for reasons of comfort, it is desirable for its relative humidity and its odoriferous molecular content to be situated at “comfort” values.
Conventional equipment for treating cabin air comprises apparatus for mixing air that has been recycled from the cabin with new air taken from outside the aircraft, the air being filtered with a conventional particle filter.
That conventional approach gives rise to several problems:
firstly, a particle filter is very poorly suited to purifying air containing various forms of microorganism; thus, the air recycled from the cabin is purified essentially only from the particle point of view and it is reinjected into the cabin without the microorganisms contained therein being eliminated to any significant extent; furthermore, those microorganisms which are indeed stopped by the filter proliferate thereon, taking nourishment from the various particles that the filter stops; it is also known that such a particle filter is unsuitable for stopping virus type microorganisms; finally, it should be observed that odoriferous molecules (cooking odors, kerosene odors, human odors, etc.) are not stopped by such a filter;
secondly, it is necessary to replace the filter in extremely regular manner;
thirdly, new air needs to be taken from outside the aircraft (and in practice air which is already relatively hot and taken from the aircraft engines) leading directly to a loss of engine power; as an example, renewing cabin air to the extent of 25% typically gives rise to a loss of engine power of about 1%; it will be understood that this goes specifically against recent trends of achieving the maximum possible savings in terms of aircraft consumption; and
finally, the fact that a large proportion of cabin air is renewed presents the drawback of delivering air into the cabin that is extremely dry; typically, air taken from the outside has relative humidity in the vicinity of 5%, whereas the comfort threshold is situated at about 40%.
To resolve this last problem, it is known to associate the cabin air treatment equipment with a humidifier, however the other problems are not resolved; on the contrary, adding water to the air for treatment can encourage the development of certain microorganisms.
Another known solution consists in causing the air taken from the cabin for recycling purposes to pass over silica gel; the air that has been dried in this way is delivered to the cabin of the aircraft to reduce condensation phenomena therein, and the recovered water is used for humidifying new air. This likewise gives rise to problems of bacteria proliferating, given that silica gel constitutes a good medium for such proliferation.
Finally, it is known that air treatment apparatus including a particle filter can be associated with an activated carbon filter which stops microorganisms and odoriferous molecules better; however that gives rise to new problems: firstly, the head losses in the air treatment circuit are significantly increased, thus making it necessary to use more powerful air-circulation fans with an undesirable increase in energy consumption on board the aircraft; secondly the effectiveness of such an activated carbon filter is very limited in time, so it needs to be changed on practically every stopover; and finally such carbon filters are known to constitute excellent media for bacterial proliferation; for these reasons, such carbon filters are practically never used.